Utility workers were still trying to piece together exactly what happened, Florida Power and Light President Armando Olivera said.

Olivera said disconnect switch failed at 1:08 p.m. at the automated substation west of Miami, and a piece of equipment that controls voltage caught fire about the same time. Neither failure by itself would have caused a widespread outage, he said.

The "initiating event" was the failure of the disconnect switch, Olivera said.

"These systems are all designed so that you can handle two contingencies," Olivera said. "If you had a switch that failed, protective devices would have isolated the problem. That did not occur today. That's the part we don't have an answer for."

Olivera said about 475,000 Florida Power and Light customers and about the same number of customers from other utilities lost power as a result. The affected region ranged from Miami to Tampa, throughout Orlando and east to Brevard County, home to Cape Canaveral and the Kennedy Space Center. See a map of the areas affected »

The substation trouble set off a sequence of events that within two to three minutes had knocked numerous power plants off-line -- including the Turkey Point nuclear power plant south of Miami. Olivera said Turkey Point's two nuclear reactors and a natural gas-powered generation unit automatically shut down when the plant's systems detected a fluctuation in the power grid. Learn more about power grids and blackouts »

"In a fraction of a second, the demand was far greater than the power plants that were on line generating electricity could handle," he said. "When you have that kind of imbalance, we have a system that kicks in and it starts turning people's lights off, essentially balancing the demand with what's available."

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Mike Stone, a Florida Department of Emergency Management spokesman, said 2 million to 3 million people were affected at the height of the outage.

Detective Robert Williams, a Miami-Dade County police spokesman, said power was out across the entire county within 20 minutes of the initial failure. Outages extended into neighboring Broward County, which includes Fort Lauderdale, and north to Palm Beach County -- a region of about 6 million people.

But Miami International Airport, which has emergency generators, reported fewer than a dozen delays and had normal electric service back on within half an hour. Schools remained in session during the blackout, said Cmdr. Charles Hurley, a spokesman for the Miami-Dade school system's police department. And Delrish Moss, a county police spokesman, said no major traffic problems had been reported.

"Most of the calls that we're getting have to do with people being stuck in elevators and things of that nature, and people concerned about what is going on." Moss told CNN. He advised people "not to panic, and if you go out, be courteous to other drivers."

"If you come to a place where there is a no light and no policemen ... be courteous," he said. "If you're not courteous, that's when accidents occur."

The outage struck as a strong cold front and scattered thunderstorms passed through the region -- including one that prompted a tornado warning for Fort Lauderdale, the National Weather Service reported.

Stan Johnson, a spokesman for the North American Electric Reliability Corporation, said a total of eight generating units were off-line across the region.

Ken Clark, a spokesman for the Nuclear Regulatory Commission, said Turkey Point's nuclear reactors are likely to remain off-line for 12 to 24 hours. Both were in "hot standby," and operators kept them in that condition without resorting to emergency diesel generators, Clark said.

In Washington, a Department of Homeland Security official said there was no indication that terrorism was behind the blackout.E-mail to a friend